On the linear actuators composed of one or more threaded nut screws or threaded male rollers, against whose thread a screw is turned having the same thread step (or a step equal to a complete multiple or fraction, whose inverse is an integer) already described in other documents (U.S. Pat. No. 3,395,385, U.S. Pat. No. 3,730,016, U.S. Pat. No. 3,756,092), the following problems have been detected:                Impossibility of guaranteeing a linear movement proportional to the number of screw revolutions;        irregular travel velocity when the axial load of the screw drops to minimum values, due to the blocking of the nut screw bearings which are not sufficiently loaded, and to the sliding of the screw on the blocked nut screw: this provokes a travel with an irregular step and at most, equal to the threading step;        low efficiency, due to the use of inadequate thread profiles;        presence of pre-loads which, inserted for the purpose of avoiding the accidental blocking of the nut screw or the roller (or the screw) because of an excessively reduced load, drastically reduce the mechanism efficiency;        hardening or blocking in the case of a screw coupled simultaneously with more than one nut screw or roller.        
In reality, the findings described in such existing documents particularly attempt to:                use the preload to resolve the problem of travel irregularity;        block the nut screw (or the screws) to obtain a dual travel velocity equal to the screw rotation velocity;        vary the travel by revolutions of the screw, in either manual or automatic mode, in step or continuous mode, in order to adapt the velocity to the load equal to the rpm of the system drive motor.        
In particular, in the document U.S. Pat. No. 3,730,016, there is a variation device for linear travel by screw revolution according to load: however this does not possess function precision, is not efficient and the under maximum loads the threading is subjected to stress in proximity to the crest, which represents the weakest zone.
Only one document WO2004/113762 A2, whose inventor is the same as the inventor of the present invention, mentions the presence of synchronisation, however, without mentioning the device that actuates the above synchronisation.
None of the known systems, lacking synchronisation, guarantee constant and accurate travel for each revolution of the control shaft, regardless of the external load and the construction and assembly tolerances of the device. None of the cited documents performs a precise analysis of the screw/nut screw coupling, the contact points, and the causes of the poor efficiency caused by the threading profile.
None of these documents mentions the effects of the preload compression between screw and nut screw.
None of the documents mentions the effect of a very slight clearance between coupled threads in order to increase efficiency.
There is no reference to any surface hardening treatment to be carried out in order to prevent anticipated contact point wear between screw and nut screw.
Presentation of the Finding.
The primary object of the present finding is to realise a device which guarantees screw travel precision, regardless of the acting load.
Another object is to realise a device which is intrinsically reliable, safe and long-lasting in time.
A further object is to realise a device that is efficient.
Such objects are achieved according to the claims, and in particular according to the characterising part of claim 1:                by means of the synchronisation of the rotation of the elements coupled on the thread, obtained in a simple, economical and reliable manner, which can be realised by means of common machine tools and known processing techniques, through a particular conformation of the thread of at least one of the two elements coupled on the thread; in fact any synchronisation system that imposes a fixed and constant ratio between the number of screw revolutions and that of the nut screw or the roller, prevents travel irregularity and fixes the travel per revolution at a known value, regardless of the loads and frictions present;        by means of a coupling system between the threads that minimise losses due to friction;        by means of a threading profile that minimises losses due friction and improves thread resistance;        by means of a surface hardening treatment that reduces losses due to friction, and wear on the coupled threads.        